The present invention relates to specific fibers suited for use as secondary reinforcement materials of cementitious matrices in cementitious composites and, more particularly, to acrylic fibers, melt spun synthetic fibers and precursor low orientation nylon fibers for use as secondary reinforcement materials in cementitious composites. The invention further relates to methods of manufacturing of such fibers via wet spinning of acrylic fibers and melt spinning of nylon and polypropylene fibers and to methods of production of the synthetic fibers reinforced cementitious composites.
More particularly, the present invention relates to improvement of the efficiency of short cut acrylic fibers for reinforcement of cementitious composites via adjustment of their friction and surface tension characteristics to obtain superior fibers dispersion in a cementitious matrix and greater reinforcement and crack arresting efficiency. The present invention further relates specifically to short cut melt spun synthetic fibers with improved efficiency for reinforcement of cementitious composites as a result of adjustment of their friction and surface tension characteristics to obtain superior fibers dispersion in the cementitious matrix and greater reinforcement and crack arresting efficiency. The present invention also specifically relates to precursor nylon fibers having a low degrees of crystallinity and orientation, low tenacity and modulus and high elongation characteristics and to upgraded precursor low orientation nylon fibers with improved efficiency for reinforcement of cementitious composites as a result of adjustment of their friction and surface tension characteristics to obtain superior fibers dispersion in the cementitious matrix and greater reinforcement and crack arresting efficiency.
As is well known in the art, cementitious composites, such as concrete, are prone to self-induced cracking, as such composites are brittle by nature. Self-induced cracks readily propagate through concrete under relatively low stresses. Thus, concrete fails in tension by progressive crack development.
The concrete's actual low tensile strength is explained by the presence of flaws (microcracks and cracks) that propagate into bigger cracks under tension. To increase the concrete durability it is, therefore, important to minimize the presence of microcracks and cracks that are distributed therein, which weakens the concrete and reduces its durability.
When a mix of concrete, or any other mix of a cementitious composite, is placed (e.g., poured, molded, layered, sprayed, etc.), the solids, e.g., aggregates, fines and cement, therein begin to settle downward due to gravity. As the solids sink, water is displaced and forced to the surface as bleedwater. Plastic shrinkage cracking of the concrete occurs when the rate of water evaporation exceeds the rate of water displacement. Shrinkage stresses associated with early volume change account for the majority of all non-structural cracks in concrete. As mentioned above, these cracks, which are formed while the concrete mix settles, affect the strength and durability of the concrete during service. Therefore, in the common practice, concrete products are watered and cooled while hardening. However, as watering concrete products while hardening does not completely eliminate microcracks and cracks formation and calls for special care, the search for concrete additives which reduce cracks formation has begun.
The use of nylon fibers in the reinforcement of concrete is set forth in U.S. Pat. No. 3,645,961. This patent discloses the use of discrete fibers to form a blast resistance concrete. Other related publications include U.S. Pat. Nos. 5,456,752; 5,399,195; 4,693,749; 4,902,347; and SU Pat. No. 1,479,618.
The presence of nylon fibers in a concrete mix alter the process of solids settlement and water bleeding, and therefore reduce the internal tensile stresses that lead to plastic shrinkage cracking during the early volume changes of the concrete while hardening. The stress-induced microcracks that begin to form are bridged and intersected by the millions of evenly distributed fibers present in the cementitious matrix, and cracks propagation is therefore halted.
Thus, nylon fibers assist in the prevention of microcracks during settling of concrete, which microcracks form flaws which, long after settling and during service, tend to develop into bigger cracks and fractions, which weaken the concrete and reduce its durability. Nevertheless, it is important to ensure that the fibers, which constitute part of the total volume of the cementitious matrix of the concrete, will not be deteriorated during service, since the loss of internal volume strength and substance will weaken the whole concrete matrix. Thus, the requirements from nylon fibers used in concrete reinforcement are (i) efficiency in reducing microcracks formation during settling and (ii) high durability, i.e., prolonged service before deteriorating.
A substantial growth in the use of technical nylon fibers for concrete and cement reinforcement has taken place since the first trials in using nylon fibers for concrete reinforcement. Both nylon 6.6 (e.g., Du Pont Type 663 and Type 665, both are distributed by Kapejo Inc.) and nylon 6 (e.g., Alliedsignal Caprolan-RC, distributed by Nycon Inc.) prepared having technical nylon properties, are used in the art of concrete production as typical concrete secondary reinforcing fibers, aimed at combating the cracking of the concrete during the early plastic stages of its settling. The term “secondary reinforcement” is commonly used in the art of concrete production to indicate a reinforcement directed at prevention or reducing cracks associated with concrete settling.
U.S. Pat. No. 6,001,476 teaches production and use of upgraded nylon fibers for secondary reinforcement of concrete and reinforced cementitious composites including the fibers. Teachings of this patent focus on a method of upgrading the strength and durability of nylon fibers, typically textile nylon fibers, to render the fibers suitable for use in secondary reinforcement of a cementitious matrix of cementitious composites, such as concrete. This prior art patent does not teach the use of low orientation precursor nylon or of acrylic fibers.
Similarly absent from the teachings of this patent is the concept of increasing the friction between fibers and the cementitious matrix as a means of improving the reinforcement properties of the fibers. However, according to the mechanical behavior models for fiber reinforced concrete, the fibers discontinuously distributed in the cement system contribute to its load carrying capacity via load transformation to the fibers by shear deformation at the fiber/matrix interface. Therefore, following basic composite materials rules of mixture, the extensional strength and stiffness of the system increase as the fibers ultimate strength and modulus values increase. Thus, the greater the fiber strength, better the load supporting capacity and the longer the maximum possible length of stress supporting fibers in the matrix. The overall effect is therefore an increased tensile strength of the system. Accordingly, typical fibers for cement systems reinforcement are of the high strength type (e.g., steel, glass, asbestos, etc.). For the same reason, synthetic fibers that have been used for reinforcement of cementitious systems (e.g., Nylon such as “Nycon RC”—characterized by modulus of 5,170 Mpa, and tenacity of 896 Mpa, Polypropylene such as “Fibrin 23” characterized by modulus of 3,500 Mpa, and tenacity of 370 Mpa, and Polacrylonitrile such as “Dolanit 10” characterized by modulus of 19,000 Mpa, and tenacity of 1,030 Mpa) against plastic shrinkage cracking that take place within the setting period of the cement, are typically designed to have higher modulus and tensile strength relative to standard textile fibers.
The failure in cementitious systems is typically initiated by tensile fracture of the matrix that yields cracks propagating throughout the system. The role of the reinforcing fibers is in crack arresting and fracture toughening.
According to recent studies (Y. Geng and C. K. Y. Leung (1996) “Microstructural Study of Fiber/Mortar Interfaces During Fiber Debonding and Pull-out”, J. Mater. Sci. 31:1285–1294 and (1997) “Fiber Reinforced Concrete”—reported by ACI Committee 544, American Concrete Institute, Ch 4, ACI 544 IR pg 39–57), the pull-out of fibers at the crack planes is the dominant factor in the reinforcement mechanism, because of the easy debonding of synthetic fibers in cementitious systems and their lower interfacial cohesion relative to the primary reinforcing, such as steel, glass and asbestos fibers.
This means that the fiber distribution in the matrix is an important parameter—controlling the required content in the matrix for crack reduction since the inter—fiber spacing contributes to the crack arresting efficiency (R. F. Zollo (1997) “Fiber reinforced concrete: an overview after thirty years of development”, Seminar 24–62 Abredeen's world of concrete, pages 12–41). Accordingly, adjusting fiber/cementitious system and interfiber frictions offers the possibility of developing new fibers for secondary reinforcement of concrete from materials typically considered unsuitable according to prior art teachings.
U.S. Pat. No. 5,989,713 teaches fiber cross-sectional geometries that increase the surface area that is available for bonding with the cementitious matrix. The greater surface area per unit weight of reinforcing fibers increases their bonding strength and the efficiency of stress transformation from the matrix to the fibers. A greater fraction of the ultimate fiber strength is thereby utilized for load carrying and crack bridging prior to debonding and pull-out of the fibers from the matrix. Teachings of this patent do not include changing the modulus of the fibers to achieve this effect nor do they include increasing dispersability of the fiber within the system during mixing.
However, in practice the pull-out stresses on the fibers are much lower than their ultimate tensile strength and are controlled by the frictional forces between the fibers and the cementitious system matrix. Therefore the fiber's high strength and stiffness requirements that are generally deemed necessary in reinforcement of hardened concrete by strong fibers (i.e., steel, glass and asbestos) are actually not necessary in the case of secondary reinforcement by synthetic fibers.
Since the fiber to aggregate coefficient of friction is of the same nature as the standard fiber-to-metal (f/m) coefficient of friction f/m may be employed as a parameter to quantify a fiber's frictional interaction with the cementitious matrix.
The fiber/cementitious system frictional forces also determine the extent of fibers mixing and distribution within the cementitious system. Greater frictional forces overcome the interfiber cohesion and spread the fibers faster in the matrix during the mixing stage of the fibers in the fresh cement slurry. Accordingly, for effective mixing of the fibers in the cementitious system, fiber with surface properties which create high f/m coefficients of friction while keeping low fiber-to-fiber (f/f) coefficients of friction and interfilament cohesiveness are desirable.
Use of nylon, acrylic and polyvinyl alcohol fibers is taught by Goldfine's U.S. Pat. No. 3,645,961 in order to meet these criteria, however this patent does not teach any lubrication or surface treatment in order to improve surface properties of fiber.
Techniques for improvement of fibers mixability in the concrete by premoisturizing are taught by WO83/00324 but these teachings are limited to olefinic fibers.
Pretreatment procedures for polyolefin fibers, including polypropylene fibers in particular, for cementitious system reinforcement have been reported in the patent literature (e.g., U.S. Pat. No. 5,399,195). This patent teaches procedures for increasing the hydrophilic nature of the hydrophobic olefinic fibers and enhancing their dispersability and compatibility with the cementitious matrix. Procedures taught by this patent do not change the frictional characteristics of the fibers.
There is thus a widely recognized need for, and it would be highly advantageous to have, specific fibers suited for use as secondary reinforcement materials in cementitious composites, methods for producing the fibers, and cementitious composites containing the fibers devoid of the above limitations.